STUDIES ON THE ANTIGLUCOCORTICOID ACTION OF PROGESTERONE IN RAT THYMOCYTES - EARLY INVITRO EFFECTS

Progesterone was found to exhibit antiglucocorticoid properties in rat thymocytes, as indicated by its ability to compete effectively for triamcinolone acetonide (TA) binding sites as well as to inhibit the biochemical effects of TA. In a cell free system, progesterone competitively inhibited the binding of TA to glucocorticoid-specific binding sites and was more effective than the antiglucocorticoid cortexolone in this respect. Progesterone at 10-7-10-6M antagonized the inhibitory effect of 5 × 10-8TA on RNA metabolism, although it was inactive by itself as an inhibitor of [3H]-uricline uptake and incorporation into thymocyte RNA; this antagonism was not due to competition by progesterone for TA uptake into the cells. Progesterone was less efficient than cortisol or TA in depleting the cytosol receptors. However, unlike the glucocorticoid agonists which formed a temperature-dependent, salt extractable nuclear bound complex, progesterone was bound tightly to a fraction in the 27,000 g pellet which was unaffected by changes in the incubation temperature or salt extraction conditions. Additional binding of progesterone was observed in cells pre-saturated with TA. suggesting the presence in rat thymocytes of progesterone binding components other than the glucocorticoid receptors. © 1979.